GCN Circular 8260
G. Stratta, M. Perri, B. Preger (ASDC) report on behalf of the Swift-XRT team: We have analyzed the first two orbits of Swift XRT data of GRB 080916B (Stratta et al., GCN Circ. 8242), consisting of 2.8 ks of Photon Counting data collected starting from 22.8 min after the burst. During the XRT observation, from T0+1.4 ks to T0+8.5 ks, the source shows a fading behaviour from 0.08 cts/s to 0.02 cts/s. Assuming a power law model, the best fit decay index is alpha = 1.0 � 0.2. At this rate we predict a count rate level of 1.7 x 10-3 cts/s one day after the trigger. The 0.3-10 keV extracted spectrum (2.8 ks of integration time) is consistent with an absorbed power law model. Fixing the column density to the Galactic value (NH = 1.8 x 1020 cm-2 from Kalberla et al. 2007) the best fit photon index is 2.0 � 0.5. The observed 0.3-10.0 keV flux is (1.0+/-0.6) x 10-12 erg cm-2 s-1 which corresponds to an unabsorbed flux of 1.1 x 10-12 erg cm-2 s-1. The count to observed flux conversion factor is 6.6 x 10-11 erg cm-2 count-1. This Circular is an official product of the Swift-XRT team.